1. Field of the Invention
The present invention relates to a method of manufacturing an electronic circuit assembly from flexible film and an injection molded plastic material. More specifically, the present invention relates to a method of attaching a plastic flexible film to an incompatible plastic material by utilizing an intermediate adhesive layer.
2. Description of the Related Arts
In-molding, insert molding or capture molding are the processes where a flexible film is molded on or into a part or substrate. The process involves the placement of a film(s) into an open mold tool cavity. The film is held in place by features of the film or the mold tool. The mold is closed and the cavity is filled with resin. The resin molds the film onto or into a structure. Typically, the molding process is injection molding but other molding techniques can be used. These in-molded films and resins are typically costly engineering thermoplastics such as polytherimide. These materials are miscible which means that the materials interdiffuse within each other during molding creating a homogeneous film-resin substrate. The film is able to be metallized via lamination and etch or pattern plating with conductive circuit traces making it an electronic substrate. The film can be fabricated with single or double layers of metallization and are metallized through conventional laminating, sputtering, evaporation and embossing, photoimaging, etching and plating processes.
Another in-molding process is called transfer in-molding. This process uses a metallized foil as described above. However, the film resin is not miscible with the molded resin. After the in-molding of the film in the mold, the film is peeled away from the molded substrate leaving the circuit traces mechanically embedded in the molded substrate.
The problem particularly with film insert molding is that there are a limited number of compatible resins and corresponding films made from the resin. These films also tend to be expensive relative to commodity automotive plastics such as ABS and polypropylene. Currently the most widely used capture molding materials system is amorphous polyetherimide (PEI) resin with polyetherimide (PEI) film. Other systems include polyethersulfone (PES), and polysulfone (PS). Typically insert molding systems use the same polymer for both the film and resin. These in-molding resins are also not typically used in any appreciable amount in automotive components.
It is known to manufacture circuit assemblies from flexible films having flexible electronic circuits printed or traced thereon. An example of commercially available films produced by this construction is illustrated in U.S. Pat. No. 5,461,202. Flexible films enable the production of electronic circuit boards into highly contoured three-dimensional shapes. They also allow the electrical circuits to be manufactured separately from the backing structure. This enables the use of different materials for the backing structure and film. One way that flexible films are used is through a process called film capture or in-molding. In-molding refers to the process where a flexible film of either metal, polymer or a combination of both is inserted into a mold and subsequently molded into a circuit assembly.
The film becomes adhered to the backing structure by either mechanical interlocking or an interdiffusion of the resin and polymer film. These films typically contain electrical wiring features or circuits to create a functional electronic module when fully assembled with electronic components.
One example of an electronic circuit made from an in-molding process is illustrated in U.S. Pat. No. 5,118,458 ('458). This reference teaches a method of producing a multi-layer circuit assembly by sequentially building up layers of flexible films. A first flexible film has a circuit traced thereon and a hot-melt adhesive is placed on the reverse side. The film is then placed within a molding tool and a resin is injected against the facing side of the film. The mold is heated to cure the circuit traces. Additional film layers are sequentially added. The hot-melt adhesive does not act to secure the film to the injected resin. Rather, the adhesive is used to attach the various circuit layers to one another.
It is also known to manufacture electronic circuit assemblies by forming a backing only along a portion of a flexible film as shown in U.S. Pat. No. 5,220,488 ('488). The '488 patent teaches placing an elongated film into a mold tool and selecting a plastic resin that fuses to the film. The '488 device does not teach the use of a resin that is incompatible with the films.
There are two major difficulties when attempting to manufacture circuit assemblies made of a flexible film and rigid plastic substrate that are made from incompatible materials. The first problem relates to lamination failure because the film does not adhere or bond to the resin. Dissimilar plastic materials do not intermingle and fuse when melted and have very poor adhesion.
Another difficulty when making the film and substrate from different materials is thermal fatigue placed on the electronic circuits from the different coefficients of thermal expansion (CTE). Different materials have different CTE. The CTE of a material is a measure of the amount of elongation and contraction it undergoes with temperature. When the film and resin are made from different materials, they expand and contract by a different amount with temperature. This difference in expansion and contraction may cause either the film or circuits placed on the film to weaken or fail over time. It is useful to provide an intermediate layer that ameliorates the different expansion and contractions.
These deficiencies and problems are overcome by the present invention.